In the digital PWM AC inverter based motor drive system, good quality of the motor phase current feedback is required to perform the closed loop current control. In order to reduce the system cost and simplify the current sensing system, the multiple motor phase current information can be reconstructed via DC link current information. The traditional circuit to achieve the motor phase current reconstruction via the DC link uses a single sample and hold circuit combined with an A/D converter. However, this method imposes a significant limitation on reconstruction when the line-to-line active voltage becomes small due to the PWM logic when the time duration of a switching pattern generating the active voltage vector is small. A new method is described herein having two parallel sample and hold circuits instead of one sample and hold circuit to minimize the time duration limitation on the active voltage vector. With this new circuit, fidelity of motor current reconstruction is improved and it expands the use of DC link current reconstruction for a wide range of AC motor applications including permanent magnet motors and induction motors.
Traditional DC link current sensing and reconstruction of the phase current uses both hardware and a software algorithm in a digital motor control application. The hardware circuit normally consists of an amplifier, sample and hold circuit, multiplexer and A/D converter. The location of the sample and hold circuit may be different (see FIGS. 1 and 2), however it has only one sample and hold circuit as a common approach. In three phase AC motor applications, two phase current information is required to be reconstructed in order to provide three AC phase currents at a time. FIG. 3 shows a typical inverter circuit with AC motor with particular current flow which corresponds to the switching pattern (100) in a PWM cycle in FIG. 4. DC link current sensing, which can be realized by a single shunt resistor R (FIG. 3) or a hall effect type current sensor, normally contains two phase current information within a PWM cycle. FIG. 4 contains the phase U and phase W current information as an example. The duration period of each of the two phase current information on the DC link varies with synchronization of particular PWM switching pattern in a PWM cycle. In FIG. 4, the switching states of the inverter switches are shown, with “1” meaning a high side switch is on. Accordingly, active voltage vector state “100” is when high side switch U1 (FIG. 3) is on (U2 is off), V1 off (V2 on), and W1 off (W2 on). This is the state of FIG. 3. As shown in FIG. 4, during switching state or active voltage vector 100, the current in the DC link is the phase current IU, which is equal to −(IV+IW).
With a conventional reconstruction circuit as shown in FIG. 1 or FIG. 2, the first phase current is captured by a sample and hold circuit S/H and subsequently converted to a digital quantity by an A/D converter. After this first phase current sampling and conversion, the second phase current is captured by the same sample and hold circuit S/H and converted by the A/D converter. Therefore, these two phase current analog-to-digital conversions take place sequentially as follows:
1) Capture the first motor phase current by sample and hold circuit
2) Convert the first motor phase current by A/D converter
3) Capture the second motor phase current by sample and hold circuit
4) Convert the second motor phase current by A/D converter.
In a typical three phase center aligned PWM modulation scheme, the above sequential event occurs twice in a PWM cycle as shown in FIG. 4. The order of each phase current follows: the first motor phase current (T1 period)→ the second motor phase current (T2 period)→ the second motor phase current (T2 period) → the first motor phase current (T1 period). The two events are separated by the zero voltage vector 111 when no current flows.
In this example, the first motor phase current represents the motor phase U current while the second motor phase current represents the motor phase W current in negative value. The phase V motor phase current is derived by the fact that the sum of all three phase motor currents equal to zero.
This conventional method of two phase motor current reconstruction has a limitation when the duration period of phase current (T1 and/or T2 in FIG. 4) becomes shorter than the time required for the sample and hold circuit and A/D converter to both capture the phase current analog value and complete A/D conversion. A typical time for the sample and hold circuit to capture the analog value is around a few hundreds of nanoseconds to one microsecond, while a typical time for A/D conversion ranges from a few microsecond to ten microseconds for a 12 bit A/D converter.
Therefore, practically three microseconds is a limitation below which current reconstruction cannot be achieved. The duration period (T1 and T2) is an exact time period of the active voltage vector. In the FIG. 3 example, they are the (100) and (110) vectors respectively for T1 and T2 periods.
As the voltage vector rotates in the space vector domain (FIG. 5), motor phase current reconstruction cannot be achieved when it reaches the so-called “sector crossing” regions shown as the hatched areas. In these regions, either T1 or T2 become very small and do not give enough time for both capturing and A/D conversion.